Re: [PATCH bpf-next] bpf/lpm_trie: inline longest_prefix_match for fastpath

[Jesper Dangaard Brouer <hawk@kernel.org>]

On 15/03/2024 18.08, Jesper Dangaard Brouer wrote:
> 
> 
> On 15/03/2024 16.03, Daniel Borkmann wrote:
>> On 3/12/24 4:17 PM, Jesper Dangaard Brouer wrote:
>>> The BPF map type LPM (Longest Prefix Match) is used heavily
>>> in production by multiple products that have BPF components.
>>> Perf data shows trie_lookup_elem() and longest_prefix_match()
>>> being part of kernels perf top.
>>
>> You mention these are heavy hitters in prod ...
>>
>>> For every level in the LPM tree trie_lookup_elem() calls out
>>> to longest_prefix_match().  The compiler is free to inline this
>>> call, but chooses not to inline, because other slowpath callers
>>> (that can be invoked via syscall) exists like trie_update_elem(),
>>> trie_delete_elem() or trie_get_next_key().
>>>
>>>   bcc/tools/funccount -Ti 1 
>>> 'trie_lookup_elem|longest_prefix_match.isra.0'
>>>   FUNC                                    COUNT
>>>   trie_lookup_elem                       664945
>>>   longest_prefix_match.isra.0           8101507
>>>
>>> Observation on a single random metal shows a factor 12 between
>>> the two functions. Given an average of 12 levels in the trie being
>>> searched.
>>>
>>> This patch force inlining longest_prefix_match(), but only for
>>> the lookup fastpath to balance object instruction size.
>>>
>>>   $ bloat-o-meter kernel/bpf/lpm_trie.o.orig-noinline 
>>> kernel/bpf/lpm_trie.o
>>>   add/remove: 1/1 grow/shrink: 1/0 up/down: 335/-4 (331)
>>>   Function                                     old     new   delta
>>>   trie_lookup_elem                             179     510    +331
>>>   __BTF_ID__struct__lpm_trie__706741             -       4      +4
>>>   __BTF_ID__struct__lpm_trie__706733             4       -      -4
>>>   Total: Before=3056, After=3387, chg +10.83%
>>
>> ... and here you quote bloat-o-meter instead. But do you also see an
>> observable perf gain in prod after this change? (No objection from my
>> side but might be good to mention here.. given if not then why do the
>> change?)
>>
> 
> I'm still waiting for more production servers to reboot into patched
> kernels.  I do have some "low-level" numbers from previous generation
> AMD servers, running kernel 6.1, which should be less affected by the
> SRSO (than our 6.6 kernels). Waiting for newer generation to get kernel
> updates, and especially 6.6 will be interesting.

There were no larger performance benefit on 6.6 it is basically the same.

Newer generation (11G) hardware latency overhead of trie_lookup_elem
  - avg 1181 nsecs for patched kernel
  - avg 1269 nsecs for non patched kernel
  - around 7% improvement or 88 ns

> 
>  From production measurements the latency overhead of trie_lookup_elem:
>   - avg 1220 nsecs for patched kernel
>   - avg 1329 nsecs for non patched kernel
>   - around 8% improvement or 109 nanosec
>   - given approx 12 calls "saved" this is 9 ns per function call
>   - for reference on Intel I measured func call to cost 1.3ns
>   - this extra overhead is caused by __x86_return_thunk().
> 

> I also see slight improvement in the graphs, but given how much
> production varies I don't want to draw conclusions yet.
> 

Still inconclusive due to variations in traffic distribution due to
load-balancing isn't perfect.

> 
>>> Details: Due to AMD mitigation for SRSO (Speculative Return Stack Overflow)
>>> these function calls have additional overhead. On newer kernels this shows
>>> up under srso_safe_ret() + srso_return_thunk(), and on older kernels (6.1)
>>> under __x86_return_thunk(). Thus, for production workloads the biggest gain
>>> comes from avoiding this mitigation overhead.
>>>
>>> Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>

I'm going to send a V2, because kernel doc processing found an issue:
  - https://netdev.bots.linux.dev/static/nipa/834681/13590144/kdoc/stderr

I'll try to incorporate the production improvements we are seeing, but
it feels wrong to write about kernel 6.1 and 6.6 improvements, but I
(currently) cannot deploy a bpf-next kernel in production (but I do test
latest kernels in my local testlab).

--Jesper